1. Field of the Invention
The presently disclosed subject matter relates to a press load controlling apparatus for a mechanical press driven by a crank or a linkage mechanism, and more particularly, to a technique of controlling a press load by a cylinder-piston mechanism provided in a slide of a mechanical press.
2. Description of the Related Art
Up to now, mechanical presses of this type have been described in Japanese Patent Application Laid-Open Nos. 2001-1199, 08-118083, and 06-155088.
An overload preventing apparatus for a mechanical press described in Japanese Patent Application Laid-Open No. 2001-1199 is provided with a hydraulic pressure chamber for overload absorption in a slide of the mechanical press, and is also provided with an overload preventing valve that performs a relief operation when the pressure of the hydraulic pressure chamber exceeds a set overload pressure. The overload preventing valve is provided with a valve-closing spring and a pneumatic cylinder that press a relief member against a relief valve seat. Compressed air having a predetermined pressure is supplied to and discharged from a pneumatic operation chamber, of the pneumatic cylinder, whereby the set overload pressure can be changed along a press capacity curve.
In the overload preventing apparatus for the mechanical press described in Japanese Patent Application Laid-Open No. 2001-1199, the compressed air having the predetermined pressure is supplied to and discharged from the pneumatic operation chamber of the pneumatic cylinder included in the overload preventing valve, whereby the set overload pressure is changed along the press capacity curve. In the case where the pressure of the hydraulic pressure chamber for overload absorption provided, in the slide of the mechanical press exceeds the set overload pressure, the overload preventing valve performs the relief operation, to thereby prevent the overload.
An overload protector for a link press described in Japanese Patent Application Laid-Open No. 08-118083 is provided with: an accumulator that adjusts the hydraulic pressure of a hydraulic pressure chamber provided in a slide; and a hydraulic pump that supplies pressure oil for hydraulic pressure adjustment to the accumulator. The hydraulic pump is controlled such that the pressure of the hydraulic pressure chamber does not exceed a predetermined value.
An applied pressure holding apparatus for a mechanical press described in Japanese Patent Application Laid-Open No. 06-155088 is provided with a hydraulic cylinder interposed, between a connecting rod, and a slide. Hydraulic oil supplied to and discharged from the hydraulic cylinder is controlled by a hydraulic pressure control mechanism, and relative motion is caused between the connecting rod and the slide. The slide is maintained at a given position whereas the connecting rod moves, and the applied pressure is held near the bottom dead center. As a result, a high-quality pressed product without unevenness in shaping can be obtained even from a material having large springback.